Flyback converters for driving LED arrays from the AC power line are well known in the art. See for example, Fairchild Semiconductor publication application note AN-9750, entitled High-Power Factor Flyback Converter for LED Driver with FL7732 PSR Controller, the disclosure of which is incorporated herein by reference, which shows a flyback controller. One of the features which is conventional in a flyback transformer design is to have close coupling between the primary winding and the secondary or output winding. This is because if there is flux leakage, as illustrated in FIG. 1, then when the primary switch is opened, energy stored in the leakage flux field must be dissipated somehow, usually as heat in a snubber arrangement. The extent to which this happens decreases the efficiency of the converter and is undesirable. There are various ways to achieve close coupling and low flux leakage, one of the most common being to interleave the primary and secondary windings in what is known as a sandwich arrangement. FIG. 2 shows a conventional flyback transformer cross section in which primary and secondary windings are alternated in order to minimize flux leakage. This illustrates how transformer designers have techniques at their disposal which allow them to make the coupling between transformer windings tighter or looser.
In a transformer which has say, windings 1 and 2, the inductance of winding 1 with winding 2 open circuit is called the primary inductance. Measuring the same inductance with winding 2 shorted measures the leakage inductance which is associated with flux through winding 1 which does not couple with winding 2. The primary inductance we shall call Lp, and the corresponding secondary inductance Ls. The coupling between the primary inductance and the secondary inductance is characterized by a mutual inductance Mps. If there was no flux leakage then Mps relates to Lp and Ls by the equation:Mps=√(Lp·Ls)
In real life there is always some flux leakage, and so the observed Mps is less, so that Mps=k√(Lp·Ls) where k is a coupling coefficient ranging from zero to unity.
Transformer engineers can control the value of k using the physical design of the transformer. In a classic flyback converter, the value of k representing the coupling between primary and secondary is always kept extremely close to unity in order to maximize the efficiency.
Since most flyback transformers are controlled by an integrated circuit chip, it is conventional to have a third winding present which is used to generate power to operate the control chip. In some flyback designs which use so called “primary side regulation” (PSR) the voltage from this winding is also used as a source of feedback to the control chip to indicate variously the voltage, current or power of the secondary side winding. Such an arrangement is shown in FIG. 3. Here the primary winding is labelled N1 (341), the secondary winding is labelled N3 (343) and the third feedback winding is labelled N2 (342). Diodes 331 and 332 divide the signal from the feedback winding N2 for the purposes of chip Vcc power and signal feedback respectively. In this arrangement the coupling coefficient K13 between winding N1 and winding N3 is close to unity, and the coupling coefficient K32 between winding N3 and winding N2 is close to unity.
The feedback signal is very important for the flyback control chip (not shown) since it represents the power from the output and is used by the control chip to compute the power factor correction process which ensures that the current drawn from the power line is sinusoidal and in phase with the power line voltage. This means that the power factor can be high (close to unity) and the harmonic distortion (THD) of the input current will be low. For most commercial purposes THD of less than 20% is required and less than 10% is considered excellent.
Also shown in FIG. 3 is a fourth winding N4 (344). The ac voltage from this winding is rectified by diode 333 and then smoothed on capacitor 334 so that it can be used as an auxiliary output source. Such a source may be used, for example, to provide power for a fan, sensors, a wireless control system, or to operate analog or PWM dimming circuitry. Such a power source is a standard ingredient for contemporary LED lighting systems, and the power drawn from it may be relatively large and variable. However, since the winding N4 is typically wound on the outside of a transformer with relatively loose coupling to the other windings, then when increasing amounts of power are drawn from it, this event is not fully reflected in the feedback signal sent back to the control chip from winding N2. The coupling coefficient K42 (coupling coefficient between windings 4 and 2) is usually relatively low. (It is difficult to simultaneously have high coupling coefficients between multiple windings at the same time, so a process of compromise is necessary.) This means that the power factor and THD of the input current will become degraded when substantial power is drawn from the auxiliary power output since the control chip is not receiving a signal which properly reflects the power being drawn from that output.
From the foregoing it is apparent that there is a need for an LED drive circuit which can provide an auxiliary power supply source, responsive to varying loads, for the use of associated circuitry, without degrading the power factor and THD of the driver and while maintaining constant drive to the main LED load.